Growing environmental awareness has developed a market need for recycling plastic articles. Such articles are made from nonrenewable petrochemical resources, consume diminishing landfill space, and decompose very slowly. The market for recycled plastic is cost-sensitive, and removing contaminants from post-consumer plastics is a major cost of processing them. Accordingly, high-speed, automated sorting systems are needed to sort foreign materials from post-consumer plastic articles.
Many post-consumer plastic articles are containers, such as beverage containers, that are of a single plastic (e.g., polyethylene terephthalate, referred to as PET) and are originally sold with an associated top or cap of a different material (e.g., aluminum or polypropylene). Consumers frequently return such containers for recycling with the top or cap still attached, thereby introducing undesirable contamination into and greatly diminishing the value of the recycled plastic.
Typically, recycling of post-consumer plastic articles includes shredding or flaking the items before subsequent processing. A conventional automated sorting system can have difficulty distinguishing certain common foreign materials from the desired plastic flakes.
More specifically, many conventional sorting systems include a white conveyor belt for carrying articles to be inspected (e.g., the flaked plastic articles) past a video camera that generates a video signal representing the articles on the conveyor belt. With many post-consumer plastic containers being of clear or translucent plastic and the contaminating polypropylene or aluminum caps being white, the sorting system is incapable of distinguishing undesirable white caps from the apparently white color of translucent plastic flakes on the white conveyor belt. As a consequence, it has been difficult to achieve high-throughput, automated sorting of foreign materials from flaked translucent post-consumer plastic articles.
Other sorting systems are directed to sorting whole plastic containers, typically one at a time. These systems either drop each whole container through an inspection zone or carry each whole container on top of a conveyor belt so the container extends between a light source and camera positioned over the conveyor belt. Both types of system suffer from relatively low throughput and are incapable of removing from the recycled articles attached foreign objects, such as caps or tops that are attached to containers.